If the work could be paid work you typically need a WP. The big issue is this is still not a viable pathway to get a WP or PR in Canada given your profile. Not sure if you realize how difficult it is to secure a WP or PR in Canada these days or the high cost of living. Securing an entry level job for a Canadian with a university degree is hard. Even if you are able to legally volunteer you would need to show you have significant savings to support yourself during that time. If you are looking for volunteer experiences would suggest you find a US based charity and consider traveling abroad (not Canada) to gain experience.
What work could not be paid work? Btw, when fire fighters from US participates in clearing wild fires in BC/ON, do they need WP/visa in order to help clear them? Likewise Canadian fire fighters and their equipments sent to clear wild fires in CA, do they need visa? Aren't firemen volunteers?